1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotography technology, such as a copying machine or a laser beam printer, and more particularly, to a control method for image formation including a step of charging a surface of the image bearing member as a member to be charged.
2. Description of the Related Art
In the image forming apparatus or the like using the electrophotography technology, a contact type charger is used, in which a voltage is applied to a charging member, and the charging member is brought into contact with the surface of the member to be charged for charging.
The contact charger is categorized into a direct current (DC) charging type and an alternative current (AC) charging type. In the DC charging type, only a DC voltage Vdc is applied as a charging bias to the charging member to charge the member to be charged. In the AC charging type, an AC voltage Vac is superimposed on the DC voltage Vdc that is applied to the charging member to charge the member to be charged. The AC charging type is more effective than the DC charging type in charging uniformly because the AC component suppresses a variation of the charging voltage. Therefore, the AC charging type is more common in recent years.
The AC charging type applies the AC voltage having a sine wave to the charging member. Therefore, the charger vibrates physically at a frequency of the sine wave, which causes vibration sound. The AC charging type applies the AC voltage having a frequency of approximately 1 kilohertz (KHz), which is within the audible frequency range for human being. Therefore, there is a problem that human being finds the generated sound uncomfortable, that is, a problem of so-called charging sound. For instance, Japanese Patent Application Laid-Open No. H05-011571 proposes a solution for this problem.
There is a problem that interference fringes appear in a visualized image after developing because of a difference between an AC frequency of the charging bias (hereinafter, referred to as a charging bias frequency) and an AC frequency of a developing bias that is applied to a developer carrier of a developing device (hereinafter, referred to as a developing bias frequency). This problem is described in Japanese Patent Application Laid-Open No. H09-101656.
In the method described in Japanese Patent Application Laid-Open No. H05-011571, the charging bias frequency is set to a lower value depending on a type of the image. Thus, the frequency of the generated charging sound is lowered to be a frequency in the range to which human sense of hearing is not sensitive, so that intensity of the generated sound is reduced.
In the method of changing the charging bias frequency according to a type of the image, the charging bias frequency can be changed when forming an image pattern causing no interference fringes, as in the case of a character pattern. However, as for an image that is apt to generate interference fringes, the charging bias frequency cannot be changed. Therefore, generation of the charging sound cannot be suppressed. Even if the charging bias frequency is changed, the frequency is constant during the image formation. Therefore, the charging sound has a constant peak frequency. In contrast, there is considered a method of reducing uncomfortable peak sound by changing the charging bias frequency within a constant range even during the image formation to realize spread spectrum of the charging sound. However, this method may generate interference fringes with respect to a formed image because of interference between the developing bias frequency applied to the developing device and the charging bias frequency applied to the charging member as described in Japanese Patent Application Laid-Open No. H09-101656.